Polyphenylene sulfide (hereinafter may be abbreviated as “PPS”) has a high melting point and favorable properties as engineering plastics, for example, excellent flame resistance, chemical resistance and the like. Because PPS can be formed into various moldings, films, sheets, fibers and the like by a common melt processing method such as extrusion molding, injection molding, or compression molding, PPS is generally used in a wide range of fields covering electrical and electronic equipment, automotive equipment and the like. However, PPS is problematic in that it generally has low melt stability, and particularly problematic in that the quality of the obtained moldings is not constant in the production of films, fibers and the like in which the melt residence time is long.
A current mainstream production process of PPS is a solution polymerization process in which an alkali metal sulfide such as sodium sulfide is allowed to react with a dihalogenated aromatic compound such as p-dichlorobenzene in an organic amide solvent such as N-methyl-2-pyrrolidone, but PPS produced by this process is problematic in that it generates a large amount of gas when heated because such PPS contains a large amount of low molecular weight components. Those gas components can cause deposits on metal molds and spinnerets during melt processing, and then it is desired to reduce the gas from the viewpoint of enhancing quality and productivity.
In response to the problem posed in that the quality of the obtained moldings is not constant, a process in which the pH of a washing fluid is adjusted when PPS obtained by a solution polymerization process is washed is proposed as a production process of PPS whose viscosity change is reduced during melt residence (JP 2005-225931 A).
In response to the problem posed in that the PPS generates a larger amount of gas when heated, a PPS obtained by heating a prepolymer containing a cyclic PPS is proposed as a PPS that generates a reduced amount of gas when heated (WO 2007/034800).
As known technologies related to WO 2007/034800, a process in which a polymerization rate is enhanced by allowing a metal carboxylate salt to coexist during the heating of a prepolymer (JP 2011-173953 A) and a process in which a PPS having a reactive terminal incorporated therein is obtained by allowing a sulfide compound having a functional group such as an amino group to coexist (WO 2013/161321) are known.
Further, a process in which a PPS having excellent heat resistance and low gas generation is obtained by blending the PPS obtained by a solution polymerization process and the PPS obtained by WO 2007/034800 (WO 2013/099234) and a process in which a PPS containing a reactive terminal group and having excellent low gas generation is obtained by blending the PPS obtained by WO 2007/034800 and the PPS obtained by WO 2013/161321 are proposed.
When PPS is obtained by a process described in JP 2005-225931 A, it has a small viscosity change during melt residence, but the effect is not sufficient, and in addition, there is a problem in that the process causes a large amount of gas to be generated by heating because it uses a PPS obtained by a solution polymerization process. On the contrary, when PPS is obtained by a process described in WO 2007/034800, it generates a smaller amount of gas when heated, but such a PPS is problematic in that it has a larger viscosity change during melt residence.
The PPSs obtained by JP 2011-173953 A and WO 2013/161321 are also unsatisfactory in terms of melt stability, and it is inferred that the reason for it is that the PPSs obtained by these processes have active terminals derived from additives and tend to cause postpolymerization and cross-linking reaction.
The PPSs obtained by processes of WO 2013/099234 and WO 2013/161321 are also unsatisfactory in terms of melt stability, and the PPSs obtained in Examples described in WO 2013/099234 and WO 2013/161321 do not allow their sufficient enhancement in melt stability to be confirmed.
It could therefore be helpful to provide a PPS having a smaller viscosity change during a long-time melt residence and generating a smaller amount of gas when heated.